The present invention is a method for cleaning a substance having the nature of a viscous liquid from a labyrinthine channel system which may contain parallel channels and dead-end zones. The method is particularly well adapted for cleaning automatic adhesive extrusion equipment between periods of use.
Systems which extrude a coating or adhesive onto a substrate materail are well known in the art. As one example, U.S. Pat. No. 4,223,633 to Alvensleben et al. shows an extrusion head for application of an adhesive to corrugated medium. Radowicz, in U.S. Pat. No. 3,938,467 shows another system for continuously mixing and applying an adhesive for wood gluing.
All of these systems must periodically be shut down and cleaned. It is normally impractical to mechanically disassemble all of the equipment used in application systems of the general type just mentioned. Cleaning is normally accomplished by flushing a solvent through the part of the system from which it is desired to remove the contained substance. An example of this is found in the aforementioned patent to Radowicz. Water is the solvent of preference but an organic solvent may be necessary when the contained substance is not water tolerant. If the viscosity of the substance being applied by the system is quite low, cleanup with a solvent such as water is usually very satisfactory. Unfortunately, substances such as adhesives which might be applied by such a system usually have relatively high viscosities or else have rheological properties which make them similar to high viscosity liquids. One example that can be cited is the application foamed phenolic resin adhesives in the continuous manufacture of plywood. A system of this type exemplified in one of the following U.S. patents to Cone and Steinberg: U.S. Pat. Nos. 3,895,984, 3,905,329, and 3,965,860. A satisfactory adhesive for use in these systems is shown by Cone and Steinberg in U.S. Pat. No. 3,905,921.
In the above noted adhesive application systems, a formulated phenolic resin mixture is foamed with air and applied through an extrusion head to wood veneer moving on a conveyor belt beneath the head. The foamer may be of the type disclosed in one of the above patents to Cone et al. or it may a type similar to those shown in patents to Jurgensen, Jr. et al. U.S. Pat. No. 2,695,246 or Oakes U.S. Pat. No. 3,081,069.
The extrusion heads may be of the type shown in the above Cone et al. patents or they may have a flow distribution system similar to those shown in the U.S. patents to Winstead U.S. Pat. No. 2,734,224 or Wells U.S. Pat. No. 3,381,336. In the latter cases the incoming fluid is divided in a series of repeatedly bifurcating channels to ensure even distribution to each of the application nozzles.
Unfortunately, adequate cleanup is not necessarily obtained in systems of the above type by simply flushing them with a liquid solvent. As an example, in extrusion heads of the types shown by Wells or Winstead the cleanup liquid may preferentially follow one series of channels leaving untouched the viscous substance in a series of parallel channels. The same is true is areas such as pumps or foamers where blind or dead-end channels occur. Surprisingly, even with the high degree of turbulence existing within a centrifugal pump, it has been found that in some designs a dead zone exists adjacent to the area where the shaft passes into the packing gland.
The present invention overcomes the problems experienced in the prior art in cleanup of systems where parallel channels or dead zones exist and it increases the efficiency of cleaning simpler systems. It is particularly useful for the cleanup of any equipment of complex internal configuration.